1. Field of the Invention
The present invention relates to the determination of petrophysical properties of geologic structures and, more particularly, to resistivity well logging using nuclear magnetic resonance.
2. Description of Related Art
Measurement of rock formation resistivity by electrode well logging is an established technique for estimating the petrophysical properties of the formation surrounding a "conducting" borehole. Numerous devices employing various arrays of electrodes are, or have been, in use since the first resistivity log was recorded in Pechelbronn, France in 1927 by Conrad Schlumberger. All such devices have in common the production of a current density field in the formation by an electric power source and the mapping of potential differences along the borehole or the mapping of electrode potentials required to maintain a specified current distribution in the borehole. See, Bassiouni, Zaki, "Theory, Measurement, and Interpretation of Well Logs"; Society of Petroleum Engineers; Richardson, Tex., 1994, Chapter 5. All prior art electrode devices measure voltages or currents at the internal surface of the borehole. Subsequent analysis is then performed to loosely correlate these borehole measurements with some of the petrophysical characteristics of the surrounding formation.
Induction tools were introduced in the mid-1940's to estimate resistivity in non-conducting boreholes. These devices magnetically induce a current flux in the formation surrounding the borehole, which formation acts as a lossy distributed mutual inductance between two or more measuring inductances. Exemplary of such a device is that shown and described in U.S. Pat. No. 5,428,293 to Sinclair et al.
While such electrode and induction tools have been widely used over the years, they have not proven to be fully satisfactory because they provide only gross approximations of resistivity distributors. Attempts have been made to overcome some of the disadvantages of both induction and of direct contact electrode current and voltage measurement devices by using other logging techniques.
Nuclear magnetic resonance devices measure other related characteristics of the rock formation surrounding a borehole. Nuclear magnetic resonance devices have been applied, for example, to measure such geophysical properties as porosity, pore size distribution, bulk fluid volume, and irreducible bound fluid volume of geological formations surrounding a borehole. Applications of this type are exemplified in U.S. Pat. No. 4,933,638 to Kenyon; No. 5,212,447 to Paltiel; No. 5,280,243 to Miller; No. 5,389,877 to Sezginer; No. 5,412,320 to Coates; No. 5,432,446 to Macinnis; No. 5,486,761 to Freedman; and No. 5,557,200 to Coates.
Representative of magnetic resonance logging tools is a device marketed under the mark MRIL by Numar Corporation. The Numar device has a sensitive volume approximating a thin cylinder 24 inches in height, 16 inches in diameter, and of one millimeter slice thickness surrounding a borehole of 8 to 12 inches diameter. This device permits measurements to be made peripheral to the borehole mud, the mudcake on the borehole wall and often to the flushed zone and the transition zone, yielding an improved estimate of the properties of the uninvaded formation surrounding the borehole relatively free of borehole effect. See, Bassiouni; op. cit. p. 71, 72.
While the value of concordant resistivity data has long been appreciated, and while the application of nuclear magnetic resonance techniques to the derivation of reasonably accurate information on pore size, bulk fluid volume and similar physical characteristics of geologic formations has been recognized, the advantages attendant to the simultaneous use of nuclear magnetic resonance for additionally determining the resistivity of geologic structures surrounding a borehole have not heretofore been realized.